Powder metallurgical techniques enable producing parts with complicated shapes in shapes extremely close to product shapes (so-called near net shapes) with high dimensional accuracy, and therefore machining costs can be significantly reduced. For this reason, powder metallurgical products are used as various mechanical structures and parts thereof in many fields.
Further, in recent years, to achieve miniaturization and reduced weight of parts, an increase in the strength of powder metallurgical products is strongly requested. In particular, there is a strong request for strengthening iron-based powder products (iron-based sintered bodies).
Generally, an iron-based powder green compact for powder metallurgy which is a former stage of an iron-based sintered body is produced by adding to an iron-based powder, an alloying powder such as copper powder and graphite powder, and a lubricant such as stearic acid and zinc stearate to obtain an iron-based mixed powder, injecting said powder into a die and performing pressing. Based on the components, iron-based powders are categorized into iron powder (e.g. pure iron powder and the like), alloy steel powder, and the like. Further, when categorized by production method, iron-based powders are categorized into atomized iron powder, reduced iron powder, and the like. Within these categories, the term “iron powder” is used with a broad meaning encompassing alloy steel powder.
The density of an iron-based powder green compact for powder metallurgy which is obtained in a general powder metallurgy process is normally around 6.8 Mg/m3 to 7.3 Mg/m3. The obtained iron-based powder green compact is then sintered to form an iron-based sintered body which in turn is further subjected to optional sizing, cutting work or the like to form a powder metallurgical product. Further, when an even higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.
Conventionally known powders with an alloying element added thereto at the stage of precursor powder include (1) mixed powder obtained by adding each alloying element powder to pure iron powder, (2) pre-alloyed steel powder obtained by completely alloying each element, (3) diffusionally adhered alloy steel powder obtained by partially diffusing each alloying element powder on the surface of pure iron powder or pre-alloyed steel powder, and the like.
The mixed powder (1) obtained by adding each alloying element powder to pure iron powder is advantageous in that high compressibility equivalent to that of pure iron powder can be obtained. However, the large segregation of each alloying element powder would cause a large variation in characteristics. Further, since the alloying elements do not sufficiently diffuse in Fe, the microstructure would remain non-uniform and the matrix would not be strengthened efficiently.
Therefore, the mixed powder obtained by adding each alloying element powder to pure iron powder could not cope with the recent requests for stabilizing characteristics and increasing strength, and the usage amount thereof is decreasing.
Further, the pre-alloyed steel powder (2) obtained by completely alloying each element is produced by atomizing molten steel, and although the matrix is strengthened by a uniform microstructure, a decrease in compressibility is caused by the action of solid solution hardening.
Further, the diffusionally adhered alloy steel powder (3) is produced by adding metal powders of each element to pure iron powder or pre-alloyed steel powder, heating the resultant powder in a non-oxidizing or reducing atmosphere, and partially diffusion bonding each metal powder on the surfaces of the pure iron powder or the pre-alloyed steel powder, and advantages of the iron-based mixed powder (1) and the pre-alloyed steel powder (2) can be combined.
Therefore, high compressibility equivalent to that of pure iron powder can be obtained while preventing segregation of alloying elements. Further, since a multi-phase where partially concentrated alloy phase is diffused is formed, the matrix may be strengthened. For these reasons, development is carried out for diffusionally adhered alloy steel powder for high strength.
As described above, high alloying is one method to enhance strength and toughness of a powder metallurgical product. However, with high alloying, the alloy steel powder which becomes the material hardens to decrease compressibility and increases the burden regarding equipment in pressing. Further, the decrease in compressibility of the alloy steel powder cancels the increase in strength through a decrease in density of the sintered body. Therefore, in order to increase the strength and toughness of powder metallurgical products, a technique is required for increasing the strength of the sintered body while minimizing the decrease in compressibility.
As a technique for increasing the strength of the sintered body while maintaining compressibility such as mentioned above, a technique of adding to the iron-based powder, alloying elements such as Ni, Cu, Mo and the like which improve hardenability, is commonly used. As an element that is effective for this purpose, for example, PTL1 (JPS6366362B) discloses a technique of adding Mo as a pre-alloyed element to the iron powder in a range that would not deteriorate compressibility (Mo: 0.1 mass % to 1.0 mass %), and diffusionally adhering, to the particle surfaces of the resultant iron powder, powders of Cu and Ni to achieve both compressibility at the time of green compacting and strength of members after sintering.
Further, PTL2 (JPS61130401A) proposes an alloy steel powder for powder metallurgy for a high strength sintered body obtained by diffusionally adhering, to the steel powder surface, two or more kinds of alloying elements, in particular Mo and Ni, or Cu in addition to said elements.
With this technique, it is further proposed that, for each diffusionally adhered element, the diffusionally adhered density with respect to fine powders of particle sizes of 44 μm or less is controlled within a range of 0.9 to 1.9 times the diffusionally adhered density with respect to the total amount of the steel powder, and it is disclosed that with a limitation to such relatively broad range, impact toughness of the sintered body is obtained.
On the other hand, Mo based alloy steel powder containing Mo as a main alloying element and not containing Ni or Cu has been proposed. For example, in PTL3 (JPH0689365B), an alloy steel powder containing Mo which is a ferrite-stabilizing element as a pre-alloy in a range of 1.5 mass % to 20 mass % is proposed to accelerate sintering by forming an α single phase of Fe having a rapid self diffusion rate. It is disclosed that, with this alloy steel powder, a sintered body with high density is obtained by applying particle size distribution and the like in the process referred to as pressure sintering, and a uniform and stable microstructure is obtained by not employing a diffusionally adhered alloying element.
Similarly, PTL4 (JP2002146403A) discloses a technique regarding an alloy steel powder for powder metallurgy containing Mo as a main alloying element. This technique proposes an alloy steel powder obtained by diffusionally adhering 0.2 mass % to 10.0 mass % of Mo on the surface of the iron-based powder containing, as a pre-alloy, 1.0 mass % or less of Mn, or less than 0.2 mass % of Mo. It is disclosed that, atomized iron powder or reduced iron powder may be used as the iron-based powder, and that the mean particle size is preferably 30 μm to 120 μm. Further, it is disclosed that the alloy steel powder not only has excellent compressibility but also enables obtaining sintered parts having high density and high strength.